THE SKY GUIDE CHALLENGE

Can you see the Horsehead Nebula, an infamously tricky dark nebula?

The Horsehead Nebula is one of the most iconic deep-sky objects there is. It’s formed by a finger of dark nebulosity projecting in front of bright emission nebula IC 434. The finger resembles the silhouette of a horse’s head, similar to the side-on profile of a classic knight chess piece. The darker nebulosity which forms the nebula is known as B33, the 33rd entry in the Barnard Catalogue of dark nebulae. Your challenge this month is to try and see the Horsehead Nebula visually.

The Horsehead Nebula’s image is found everywhere – in books, online and on wall posters. However, despite deep-sky images showing its background curtain as bright, visually the nebula is extremely dim. A 12-inch reflector is probably the minimum you need to see it from typical UK skies, but smaller instruments may return a view of the nebula from darker sky locations. The use of a hydrogen-beta (H-beta) filter is also highly recommended.

To get started, first look for NGC 2024, the Flame Nebula, located east of Alnitak (Zeta (z) Orionis). It’s so close to Alnitak that the star’s glare makes it tricky to see. If you can see NGC 2024, that’s the first hurdle passed; if you can’t, then it’s unlikely that you’re going to be able to see the Horsehead.

The two key stars needed to locate the Horsehead are mag. +7.5 HIP 26756 and HIP 26820, the latter a tight pair of mag. +6.4 and +7.6 stars separated by 0.7 arcseconds. The brighter edge of the curtain of nebulosity IC 434, forming the backdrop for B33, runs between HIP 26756 and HIP 26820.

Using our chart, locate mag. +7.8 HIP 26816 just to the east of HIP 26756.

A reflection nebula, NGC 2023, surrounds HIP 26816. Without the filter, look to see whether you can get a hint of it. If so, you’re all set for the Horsehead attempt. If not, you’re unlikely to succeed with the main challenge.

You’ll need dark skies and your eyes will need to be properly dark-adapted, meaning no light at all for at least 20 minutes. If you place a black cloth over your head that may also help. Just take your time and look for the faintest hint of nebulosity between HIP 26756 and HIP 26820. This can be very hard, so give your eyes time to get accustomed to the view. Averted vision with the aid of an H-beta filter is the best way forward here. If you can see it, the Horsehead appears as a tiny dark notch approximately onequarter the way from HIP 26756 towards HIP 26820.

Make no mistake, this is a very tricky challenge. Light pollution will almost certainly render the Horsehead Nebula invisible. However, if you do manage to see and record it, please let us know.

The Horsehead Nebula is one of the most iconic deep-sky objects there is